Thermal spraying was initiated as early as 1910 when a stream of molten metal was poured into the path of a high pressure gas jet causing metal droplets to spray in a conical pattern onto an adjacent substrate to immediately freeze and form a coating of deformed particles in a lamella structure. Today, there are essentially two types of thermal spraying that use wire feedstock: combustion flame spraying and electric arc spraying.
In electric-arc (two wire) spray coating, two consumable wires form electrodes of an electric arc or "arc ball". The two wires are electrically energized and converge at a point in which the electric arc is formed. A stream of compressed atomizing gas is passed through the converging point to atomize the molten material and drive a molten metal particle stream formed by the electric arc along an axis forward of the converging zone.
Various prior patents discuss electric-arc spray systems, noteworthy of which include U.S. Pat. No. 1,968,992 (apparatus for coating surfaces), U.S. Pat. No. 2,610,092 (spray discharge nozzle), U.S. Pat. No. 4,464,414 (method for spraying metallic coatings), U.S. Pat. No. 4,992,337 (electric arc spraying of reactive metals); U.S. Pat. No. 5,066,513 (method of producing titanium nitride coatings by electric arc thermal spray); U.S. Pat. No. 4,937,417 (metal spraying apparatus); U.S. Pat. No. 4,98,557 (method of arc spraying); U.S. Pat. No. 4,986,477 (spray gun with adjustment of the shape of the jet); U.S. Pat. No. 4,992,337 (electric arc spraying of reactive metals); U.S. Pat. No. 5,017,757 (pulsed arc welding machine); U.S. Pat. No. 5,109,150 (open-arc plasma wire spray method and apparatus); U.S. Pat. No. 5,143,139 (spray deposition method and apparatus); U.S. Pat. No. 5,145,710 (method and apparatus for applying a metallic coating to threaded end sections or plastic pipes and resulting pipe); U.S. Pat. No. 5,148,990 (adjustable arc spray and rotary stream sprinkler); U.S. Pat. No. 5,191,186 (narrow beam arc spray device), U.S. Pat. No. 5,194,304 (method of thermally spraying solid lubricant onto a metal target), U.S. Pat. No. 5, 442,153 (high velocity electric-arc spray apparatus and method of forming materials); U.S. Pat. No. 5,466,906 (process for coating automotive engine blocks) and U.S. Pat. No. 5,468,295 (apparatus and method for thermal spray coating of interior surfaces).
In combustion flame spraying, high velocity oxygen fuel (HVOF) flame spraying is a method of applying materials to a variety of heat resistant surfaces. Developed on or about 1981, HVOF has proven to be a highly efficient and effective method of coating, relying upon exit gas velocities of about 4,000 to 5,000 feet per second. The process required burning fuels such as propylene or kerosene with oxygen under high pressures up to about 300 pounds per square inch in an internal combustion chamber. Hot exhaust gases discharge from the combustion chamber through exhaust ports and expand into an extended nozzle. Powders of metals or ceramic materials are fed into the nozzle and confined by the exhaust gas stream until the particle exits at the nozzle in a high speed jet stream. The particle jet stream produces a more dense coating than coatings produced with low velocity powder flame spraying techniques. Recently, in U.S. Pat. No. 5,285,967, HVOF guns have been disclosed which are said to produce a high speed gas velocity and high speed particle velocity of from about 1000 to 1800 feet per second, while simultaneously producing a low temperature gas stream having an adjustable powder feed and temperature range from about 150.degree. F. to 750.degree. F. to properly preplasticize polymers and obtain optimal temperature for the thermoplastic polymer melt.
In all of the above prior art designs, however, a single material has been routinely employed for both the barrel and nozzle chamber of the guns. For example, copper, due to its high thermal conductivity, is currently one of the most popular materials for both the barrel and nozzle sections of many HVOF water-cooled designs. Copper is desirable from the point of view that with such high thermal conductivity, cooling will be more efficient, and problems of overheating will be avoided. In addition, as the outer surface of the nozzle is sealed with o-rings to a water cooling jacket, copper, due to its high thermal conductivity, will not melt the o-rings, thereby preventing down-time and refitting of o-ring seals.
However, the high thermal conductivity of copper comes at a price. That is, copper is relatively soft, and wear problems are common, particularly when making use of carbides and other hard powders for the resulting thermal spray. In addition, the high thermal conductivity of copper results in a relatively low inner surface temperatures in either the gun chamber or nozzle section. This corresponds to low thermal efficiency of the overall process due to such heat losses, resulting in low deposition rates and deposition efficiency as well as limiting the ability to spray high temperature materials.
The deficiencies of copper have been considered and gave rise to the us of high temperature alloys (stainless steel, nickel, nickel based super alloys, etc.). These materials have lower thermal conductivity than copper and higher wear resistance. From this standpoint they are more attractive than copper and would allow for higher temperatures to be realized on the inner walls of the barrel and nozzle. However, high temperature alloys also introduce some additional problems.
First, high temperature alloys still require one to manufacture relatively thick walls (not less than 0.635 cm) to create the proper thermal resistance for heal, transfer of the combustion products to the outer cooling (water) jacket. However, the low thermal conductivity of these materials give rise to problems with the o-rings attached to the outer surfaces thereof. That is, due to the low thermal conductivity, the high temperature alloys can overheat the o-rings and cause o-ring failure.
Finally, ceramic materials have also been considered. These materials typically have lower thermal conductivities than copper, with better wear resistance. In addition, ceramics offer higher working temperatures than high temperature alloys. However, once again, due to the relatively low thermal conductivities of these materials compared to copper, ceramic materials have similar problems associated and reviewed above. In addition, ceramic materials have their own peculiar problems, such as being relatively brittle. Furthermore, as the thermal expansion properties of ceramics are different than that of the surrounding metal components, cracks are commonly observed in the heating cycle thereby further complicating HVOF design.
Accordingly, as can be see from the above, there has been a long-standing need to improve the thermal efficiency of the HVOF apparatus and process, while at the same time providing increased durability of the hardware employed therein. That being the case, it is a primary object of the present invention to develop a HVOF gun design and process that insures lower heat loss along with high durability, thereby offering higher deposition rates and deposition efficiency, as well as the ability to spray higher temperature materials, than has been previously available in the HVOF designs of the prior art.